1. Field of the Invention
The present invention relates generally to a disk drive controller, and in particular to an apparatus and method for accessing data on a computer disk storage medium by a disk drive controller, in order to reduce the load and processing requirements of a processor responsible for controlling the disk drive controller.
2. Description of the Related Art
One of the more significant advances seen in digital data recording mediums is the remarkable increase in the recording densities available with conventional computer disks. Improvements along these lines have occasioned a noticeable shift from the use of magnetic recording mediums to that of optical disks. In great part this is due to the increased density with which optical disks store digital data.
Various formats used to facilitate optical disk data storage and access have been proposed to even further increase the recording density of optical storage mediums. One such format utilizes a zone constant angular velocity (ZCAV) system. Devices currently available that utilize ZCAV formatted data, however, require increased precision and control in their read/write operations and a higher speed in their read operations than do data access devices designed for conventionally formatted data.
The format of the ZCAV system for a disk such as that shown in FIG. 1 has the following characteristics:
(I) Data recording density increases from one zone 41 to the next, as the zones progress toward the peripheral portion of the disk. A zone 41 is an area of the disk that has the same recording density. The increase in recording density is characteristically stepwise rather than linear. PA1 (II) The heads of respective sectors 42 are randomly arranged rather than aligned together in a radial direction. The length of the sectors 42 varies from zone to zone, as for example, from zone 41a to zone 41b. The length of the sectors 42 in each zone is however constant.
FIG. 2 schematically shows a recording format of each sector 42. Each sector is divided into an ID portion 42a and a data portion 42b.
The ID portion 42a includes a sector mark portion or area (SM) 43a, a first lock-up pattern portion (first VFO) 43b, a first address mark portion (first AM) 43c, a first physical address (first ID) 43d, a second lock-up pattern portion (second VFO) 43e, a second address mark portion (second AM) 43f, a second physical address (second ID) 43g, a third lock-up pattern portion (third VFO) 43h, a third address mark portion (third AM) 43i, a third physical address (third ID) 43j, a fourth lock-up pattern portion (fourth VFO) 43k, a post ample portion (PA) 43l, an optical-system offset portion (ODF) 43m, a fifth lock-up pattern portion (fifth VFO) 43n, and a synchronizing pattern portion (SYNC) 43o.
The data portion 42b includes a plurality of user data portions 44a, a cyclic redundancy code portion (CRC) 44b, error correction code portions (ECC) 44c, resynchronizing pattern portions (RESYNC) 44d, a post ample portion (PA) 44e, and a buffer portion (BUFF) 44f. Each resynchronizing pattern portion (RESYNC) 44d is interposed among the respective user data portions 44a, the CRC portion 44b and the ECC portions 44c.
For optical disc devices using an optical disc 40 conforming to the recording format shown in FIGS. 1 and 2, data read/write operations are carried out while varying the rotational speed (angular velocity) of the optical disc 40 in accordance with the speed assigned to particular zone being searched. For example, access to the individual sectors 42 in the zone 41 is carried out at an angular velocity particular to the zone in which the sectors are located. Consequently, the angular velocity with which the optic rotates during data access varies from zone to zone.
Data read operations from one sector 42 are carried out as follows. The optical disc device first manipulates the drive head to read data from a sector. Specifically, the drive head detects a sector mark recorded on the SM 43a in the ID portion 42a for a particular collection of read data. The drive head also detects each address mark recorded on the first to third AMs 43c, 43f and 43i. Next, the optical disc device reads the respective physical addresses of the first to third IDs 43d, 43g and 43j in order to perform an error check on the read value with CRC. After the error check, the disk device determines whether to read the data recorded in the data portion 42b. The disk device next determines whether the read sector is a target sector by referring to the physical address. If a particular sector is determined to be the desired target sector, data reading operations proceed.
In order to read data recorded in the target data portion 42b, the synchronizing pattern of the SYNC 43o is first detected. Thereafter, individual pieces of data are read from the user data portion 44a at the head of the data portion 42b. The data in the residual respective user data portions 44a, CRCs 44b and ECCs 44 are read while referring to the resynchronizing patterns of the respective RESYNCs 44d.
When data is to be written in the user data portion 42b, a pattern which synchronizes the write data is written in the fifth VFO 43n. Likewise a synchronizing pattern indicative of the beginning of a data series is written into the SYNC 43o. Subsequently, data are written in each of the user data portions 44a, and a resynchronizing pattern indicative of the start of a data segment is written into each RESYNC 44d. A cyclic redundancy code is then written into the CRC portion 44b, and an error correction code is written into the ECC portion 44c.
Detection of the various patterns described above as well as data read/write operations are carried out by a detection circuit, a read circuit and a write circuit installed in the format controller of the disc drive device. The format controller is equipped with a format counter (FMC) that counts the number of pulses of a system clock signal from a frequency synthesizer. The detection circuit, the read circuit and the write circuit are controlled based on the count value determined by the format counter (FMC). The count value of the FMC corresponds to the number of bytes in one sector and is used to determine the timing of individual circuits installed in the format controller. The period of time for accessing data in each sector is called a "detection window".
In the individual circuits of the format controller, the data read/write operations undertaken for the first to third address marks 43c, 43f and 43i, SYNC 43o, RESYNC 44d, the first to third IDs 43d, 43g and 43j and user data portions 44a in a sector 42 are carried out in synchronism with the timing signals produced by the read clocks. These timing signals are generated based on the data recorded in the first to fifth VFOs 43b, 43e, 43h, 43k and 43n of sector 42.
The read clock timing signals and the system clock for the FMC are asynchronous with each other, in as much as they operate at different frequencies. This allows the count value of the FMC to be corrected every time the circuits of the format controller perform a read operation of the first to the third AMs 43c, 43f and 43i, SYNC 43o, RESYNC 44d and from the first to third IDs 43d, 43g and 43j. Lack of compliance between the count value of the FMC and the position of the drive head in a sector, caused by slippage (i.e., a lag in timing) between the read clock and the system clock, can in this way be corrected.
Occasionally, however, the individual circuits of the format controller cannot detect a mark within the circuit's operational timing period due to a pattern abnormality in the sector 42. As a result, reading or detecting the physical address cannot be completed within a prescribed period of time. When this happens, failures in mark detection and in physical address reading operations are determined. Likewise, detection of the adjacent sector 42 and read operations for that adjacent sector are carried out. If address detection and read operations are successfully preformed at an adjacent sector, the count value of the FMC is resynchronized. With this operation, the count value of the FMC is synchronized for the position of the drive head despite the occurrence of a time lag occurring between the system and read clocks.
Conventional optical disc devices, compliant with ZCAV system formatting, have the following disadvantages when performing data read operations.
As shown in FIG. 1, the heads of individual sectors belonging to a zone 41 are not arranged on a radial line extending from the center of the disc. Consequently, when a drive head seeks from a sector 42A (track number T+3; sector number K) to a sector 42B (track number T+5; sector number L) in the zone 41a, for example, slippage frequently occurs between the position of the drive head at the sector 42B and the count value of the FMC. The maximum value of the slippage corresponds to the length of one sector.
This slippage induces a time lag in the operational timing (i.e., the detection window) of the individual circuits in the format controller. Due to the time lag, the detection of each mark in a particular sector (in this case, the sector 42B of the sector number L) and reading of the physical addresses of the first to third IDs 43d, 43g and 43j cannot be performed. As a result, for sectors subsequent to Sector 42B, the mark and physical address cannot be read. This in turn breaks the synchronization between the FMC and the other internal circuits of the format controller, causing the current position of the drive head to become unidentified.
The same disadvantage occurs when plural sectors are continuously accessed over adjacent zones, for example, when the drive head is shifted from a sector 42C (track T+6; sector number J) in the zone 41a to a sector 42D (track T+7; sector number 0) in an adjacent zone 41b.
The FMC counts the number of pulses of the system clock signal from the frequency synthesizer. For every occurrence of a zone change, the frequency of the system clock signal changes to the frequency appropriate to the new zone. The frequency synthesizer generating the system clock signal needs time to change and lock the signal from its current frequency to that of a new frequency. Therefore, when the drive head shifts from a first zone 41a to a second zone 41b, the FMC carries out its count operation using the system clock signal set at the previous frequency until the system clock signal locks onto the new frequency. When this happens, the FMC carries out its count operation with the system clock set at a frequency appropriate to the new zone 41b.
In this operation, even though the timing for reading sector mark data 43a is synchronized to that of the sector head, the position of the drive head and the count value of the FMC will not coincide when the drive head passes over the sector 42. This prevents mark detection at the target sector during the physical address seek and read operations for the first to third IDs 43d, 43g and 43j. Consequently, the marks and physical addresses of all the sectors 42 subsequently sought by the drive head cannot be read. As a result, the synchronization between the FMC and the other internal circuits is lost and the target sought by the drive head remains unspecified.
In order to recognize an unspecified target for the drive head, the operation of the format controller must be temporarily stopped to resynchronize the count value of the FMC and the position of the drive head. Resynchronization control is carried out by the microprocessing unit (MPU) responsible for controlling the optical disc device. The above described resynchronization operations, as a result, increase the load of the MPU, and induce a drive delay know as "kick back". This delay degrades the performance of the optical disc device.
The same problem occurs even when, in order to access the sector 42D (track T+7; sector number 0) in the zone 41b, the frequency of the basic clock signal is locked to the frequency for the zone 41b before the drive head passes over the sector 42C (track T+6; sector number J) in the zone 41a.
Even though timing operations to read the sector mark 43b at the head of the sector 42C (sector number J) can be synchronized, slippage occurring between the drive head position and the FMC count value during the time the drive head passes over the sector 42C, prevents the drive head from reading the physical addresses of the first to third IDs 43d, 43g and 43j, etc. of the sector 42C. As a result, the disk device cannot read the sector mark 43a of sector 42D (sector number 0) in the subsequent zone 41b. The disk device, likewise, cannot read the contents of the first to third IDs 43d, 43g and 43j. This not only prevents a new sector from being accessed, but also prevents all subsequent marks and physical addresses in sector 42 from being read. Data, therefore, cannot be read out from the data portion 42b of each sector.
The inability of the disk device to carry out read operations in sectors arranged at the head area of each zone 41 increases the load of the MPU. In view of the foregoing, some formatting systems do not utilize the sectors arranged at the head area of each zone 41. This approach, however, reduces the amount of data to be recorded on the recording medium, and degrades the performance of the optical disc device.